Both analog and digital integrated circuits have been widely used in the electronics industry for many years. Until recently, however, integrated circuits were typically either digital or analog, but not both. Mixed-signal integration, in which analog and digital circuitry are integrated on the same chip, has only recently gained widespread use in response to the need to find new ways to continue the trend of increasing integration. Reducing the number of chips in a product simplifies manufacture, reduces cost and increases reliability.
Mixed-signal integration, however, presents considerable problems not present in either analog or digital integrated circuits. Notably, the noise spectrum produced by high-speed digital circuits can easily interfere with analog components. Since the waveforms transitions generated by digital circuits are, at least ideally, step transitions having (in accordance with Fourier analysis) a wide noise bandwidth, potential interference of the chip's digital signals with the chip's analog signals poses a distinct threat to circuit performance.
Generally, digital circuits switch quickly between predefined voltage levels (typically 0 volts and 3 or 5 volts), and consequently induce transient disturbances in signal and power lines, as well as energy radiated as electromagnetic waves. Digital circuits themselves are robust in the presence of noise from other sources. By contrast, analog circuits operate at a multiplicity of voltage levels and frequencies, and are sensitive to induced noise. Thus, ensuring that digital switching noise does not create interference with analog functions becomes a design challenge.
FIG. 1 illustrates a circuit example of how an analog circuit may be sensitive to the noise generated by digital circuits. In FIG. 1, integrated circuit board 10 comprises a chip 12 mounted on a package 14. The chip 12 comprises a variety of components mounted on a substrate (not shown).
The chip 12 comprises an analog circuitry 18 having an analog ground 24. The chip 12 also comprises digital circuitry (current source 20) having a digital ground 26. The analog ground 24 and digital ground 26 are coupled together (indirectly or directly) via a substrate connection 22. In some instances, the substrate connection 22 is an actual wired connection. In other instances, even though a direct connection may not exist between analog ground 24 and digital ground 26, an indirect connection 22 exists because the substrate of chip 12 itself is not perfectly insulated and acts like a connecting medium for the analog ground 24 and the digital ground 26. According to typical mixed-signal manufacturing specifications, the analog ground 24 and the digital ground 26 must be physically connected to the substrate located in the chip 12 to facilitate the functionality of the chip 12. Thus, an indirect substrate connection 22 is created via the substrate of the chip 12.
The analog ground 24 is connected to a paddle 30 located on package 14. The paddle 30 is located off of the chip 12 and is directly connected to a pad 28 located on the chip 12. In a typical case, the pad 28 is embedded in the chip 12 and the paddle 30 acts as an access point for the pad 28 and other nodes embedded in chip 12. The paddle 30 is connected to the pad 28 via a bonded wire connection 32. The pad 28 is directly connected to the analog ground 24.
The integrated circuit board 10 also has another ground node known as a board ground 34. In a typical case, the board ground 34 is created by connecting it to the ground terminal of a power source (e.g., a battery, not shown). The paddle 30 is directly connected to the board ground 34 via a bonded-wire connection 36.
The digital circuitry 20 creates digital noise as it switches between the predefined voltage levels. This digital noise comprises transient currents in the digital section of the chip 12, thereby making the digital ground 26 noisy as well. This digital noise is passed to the analog ground 24 via the substrate connection 22.
The analog ground 24 is directly connected to the pad 28. The transient currents are passed from the analog ground 24 to the pad 28. The pad 28 is directly connected to the paddle 30, thus, transient currents are passed from the pad 28 to paddle 30, thereby making paddle 30 noisy.
The board ground 34 is comparatively larger in size and is located outside the package 14 and is relatively unaffected by this digital noise. The board ground 34 is usually connected to a ground terminal of a power supply source thus, not as susceptible to the transient currents as pad 28. On the contrary the pad 28, which is located on the chip 12, is relatively noisy and affects the voltage levels and frequencies of analog circuitry 18. The pad 28 (which is an interface point to the analog ground 24) is commonly used in many analog functions (e.g., to measure voltage levels and frequencies). The noise on the pad 28 affects the correct measurement of the voltage levels and frequencies in analog circuitry 18.
One measure used in the prior art to reduce such noise interference is to group analog components (commonly termed I/O cells) together and to place analog circuitry having critical performance requirements in the vicinity of the analog I/O cells. This solution attempts to, in effect, separate the analog and digital portions of the chip to reduce the proximity of certain analog components to digital components and hence the susceptibility of the analog components to noise from the digital components.
In many systems, digital circuits switch rapidly, but regularly, with edges synchronous to a master clock and, therefore, generate noise with a strong spectral component at the clock frequency. Additionally, harmonics at odd multiple of the clock frequency are generated. If the circuit remains synchronous to a master clock, but switches on random clock edges, spectral components above and below the clock frequency also are generated. In these systems, it is possible to render the analog circuitry less sensitive to noise at the clock frequency by arranging for the analog circuits to operate on a clocked basis. Specifically, benefits are obtained if the analog circuits are clocked out of phase with the digital circuits. Alternately, the digital clock frequency may be chosen to be substantially above or below the frequency band in which the analog circuits operate.
The foregoing techniques are often not fully effective, since the digital noise can extend well above and below the clock frequency. In such situations, it is then necessary to resort to costly shielding techniques. In fact, integration of the analog and digital circuits may become infeasible, requiring that they be partitioned into separate chips, with the attendant increased cost overhead in packaging and connectors. Therefore, there remains a need for further measures to effectively reduce the digital noise in mixed-signal integrated circuits.